The present invention is a tether having the special technical feature of multiple primary load-bearing lines and normally slack secondary lines. These primary and secondary lines are connected together with knotless, slipless interconnections so the tether maintains high strength and some of the lines can be cut without failure of the tether. This type of failure resistant tether may be safely operated near the ultimate failure load of the material from which it is constructed. Such high strength, fail-safe tethers have industrial utility both on Earth and in harsh environments, such as outer space, where a single-line tether experiences a substantial risk of being cut by debris impact.
These tethers"" structures, methods of making, deploying and using them; including the specific industrial applications of an electrodynamic tether system to deorbit satellites and a low Earth orbit to lunar surface tether transport system are all part of the general innovative concept of the invention.
This application is subject to an assignment of certain rights to Gary Pansey as defined by an agreement between Gary Pansey and Tethers Unlimited dated Jul. 4, 1996. This application may also be subject to certain rights of the U.S. government as a result of contracts between the U.S. government and the inventors.
The background art includes tether structures, knotless, slipless connections between lines, tether fabrication and deployment, and space systems using tethers.
Tethers
A tether was originally a rope or chain used to fasten an animal so that it grazed only within certain limits. However, there are many specialized tether uses including bungie jumping and weather balloons. Short tethers have been used for decades in space to attach astronauts to their spacecraft.
In 1974 Professor Guiseppe Colombo, holder of the Galileo chair of astronomy at the University of Padua in Italy, proposed using a long tether to support a satellite from an orbiting platform. U.S. Pat. No. 4,097,010, which issued to Professor Colombo and Mario Grossi on Jun. 27, 1978, teaches a satellite connected by means of a long tether to a powered spacecraft. Colombo actively pursued the design of a tethered satellite system.
Several NASA experiments, such as the two Small Expendable Deployer System (SEDS 1 and 2)) and the Plasma Motor Generator (PMG) used tethers in space. SEDS used a nonconducting tether. The PMG used a 500-meter conducting tether. The Tethered Satellite System flights in 1992 and 1996 (TSS-1 and 1R) used a 22,000-meter conducting tether.
On the TSS-1 mission the tether deployed only 260 meters (853 feet) before the deployer failed. On the TSS-1R the tether was deployed 17,000 meters. In the SEDS-2 flight, a 0.8-mm diameter, 20,000-meter long braided single-line tether was deployed to study tether dynamics and lifetime. Orbital debris or a meteoroid severed this tether in less than four days.
Electric potential is generated across a conductive tether in motion across the Earth""s magnetic field lines. Electromagnetic forces acting on a conductive tether in orbit can make the tether system behave like an electric motor or generator, to exert useful force to alter the state vector of any mass attached to the tether.
In the TSS-1R flight, the conducting single-line tether was severed after five hours of deployment. This failure was caused by an electric arc produced by the 3,500 volts of electric potential generated by the conductive tether""s movement through the Earth""s magnetic field.
The Tether Physics and Survivability (TiPS) satellite consists of two end masses connected by a 4,000-meter long non-conducting tether. This satellite was deployed on Jun. 20, 1996 at an altitude of 1,022 kilometers (552 nautical miles). Its tether is an outer layer of spectra 1000 braid over a core of acrylic yarn. The yam will xe2x80x9cpuffxe2x80x9d its outer braid to two millimeters to xe2x80x9cgive it a larger cross section to improve its resistance to debris and small micrometeoroidsxe2x80x9d, according to the National Reconnaissance Office (NRO), which is a sponsor of the TiPS mission. As of Jan. 13, 1997 the TiPS tether had survived 207 days.
These experiments, all using single line tethers, illustrate the problem that long-duration and high-value tether missions require a tether capable of surviving single-point failures due to space impactors, electric arcing, or localized material defects.
In 1991, one of the present inventors, Robert Hoyt, invented a lightweight net-like structure that provides many redundant load-bearing paths. A number of primary load bearing lines running the length of the structure are connected periodically by diagonal secondary lines. Where the secondary lines intersect the primary lines, they are firmly connected by knots. The secondary lines are connected only to the primary lines. At either end of the structure, a support ring enforces the cylindrical spacing between the primary lines. The secondary lines are designed with a small amount of slack. These secondary lines are only put under load if a primary line fails. This invention was disclosed in 1992 (Forward, R. L., xe2x80x9cFailsafe Multistrand Tether Structures for Space Propulsionxe2x80x9d, AIAA paper 92-3214, 28th Joint Propulsion Conference, Nashville, Tenn., 1992 (hereinafter xe2x80x9c1992 AIAA Documentxe2x80x9d). This structure was named a xe2x80x9cHoytetherxe2x80x9d. The term xe2x80x9cHoytetherxe2x80x9d is used throughout the remainder of this disclosure for this type of structure.
The 1992 Hoytether design teaches that the normally slack secondary lines have half the cross-section (0.707 the diameter) of the primary lines. There are twice as many secondary lines as primary lines, thus the mass of the secondary lines is equal to the mass of the primary lines. In an undamaged Hoytether, the primary lines carry the entire load, while none of the secondary lines are under load.
While the survival probability of a single-line tether decreases exponentially with time, the Hoytether can maintain a high, i.e. greater than 99 percent, survival probability for periods of months or years (Forward and Hoyt, Paper AIAA 95-2890, 31st Joint Propulsion Conference, July 1995).
Slipless High Strength Line Connections
The Hoytethers and apparatus that use Hoytethers taught by the present invention require that the primary lines be firmly connected to the secondary lines, without slipping, at their crossover )unctions. The original Hoytether design did not teach the use of slipless connections. It used knots to connect the primary lines to the secondary lines. Knots cause stress concentrations, which limit the ultimate strength of the overall tether structure. It is essential that the interconnections between the primary and secondary lines be slipless to ensure proper redistribution of the loads when a segment of a primary line fails. The secondary lines in the original Hoytether design were not connected to each other.
Manufacturing companies that make fishing nets use three different knotless, slipless methods of interconnecting two lines. One is used for braided lines; one for twisted lines and one for crocheted lines. Hoytether uses lines braided of three or more smaller lines. These braids must be interwoven where they cross each other to form knotless, slipless interconnections. Alternatively the Hoytether lines may be knitted together. There is a lack of references in the field of braiding. Most of them are instruction books for hand braiding. Appendix B of the Final Report of NASA Contract NAS8-40545 (hereinafter xe2x80x98NASA Report-1995xe2x80x99, which is incorporated by reference), written by the present inventors, provides a summary of braiding fundamentals at pages B-1 to B-5. The information in NASA Report-1995 is proprietary in accordance with FAR 52.227-20 until Jun. 14, 1999. No prior art known to the inventors teaches a method of slipless knotless interconnection between a larger diameter primary line and two smaller diameter secondary lines.
Tether Fabrication
The Hoytether is essentially a tri-axial net structure, with xe2x80x98primaryxe2x80x99 lines running along the length of the tether and two sets of xe2x80x98secondaryxe2x80x99 lines connecting these primaries diagonally. They can be made by hand and connected with knots. Because knotted connections severely limit the strength of a structure, it is desirable to use a knotless fabrication technique to achieve interconnections that have strengths approaching the limits of the constituent material. As these tethers may be many kilometers long, fast and inexpensive mechanical methods are required for their practical fabrication.
The present invention may be made by mechanical braiding, i.e. three-dimensional braiding, such as 3-D rotation braiding using braiding machines such as those developed by the Herzog Company in Germany (August Herzog Maschinenfabrik GmbH and Co., Postfach 2260.26012, Oldenburg, Germany. The specialized loom developed by the Nichimo Company of Japan (Nichimo Company Ltd., 2-6-2 Ohtemachi, Chiyoda-Ku, Tokyo, Japan) is used to produce xe2x80x9cUltracrossxe2x80x9d knotless fishing nets in which the individual strands are braided as a 4-braid line, and the strands are interbraided where they cross. This produces netting that has slipless interconnections that are very strong, approaching the maximum capability of the fiber. Such a loom could, with some modifications, produce the present invention""s structure. Only two such machines exist, one in Japan, the other in Washington State. Unfortunately neither can work with the small line diameters needed to practice the preferred embodiment of the present invention. See generally, Ko, F. K., xe2x80x9cBraidingxe2x80x9d, in Engineered Materials Handbook, Vol. 1 Composites. ASM International, Metals Park, Ohio, 1957. Pp. 519-528.
The most common 3-dimensional braiding machines are 4-step braiders based upon the designs of Maistre (German Patent P230-16986, issued 1973) and Forentine (U.S. Pat. No. 4,312,261, issued 1982). Braiding is accomplished by using pneumatics or solenoids to push the parts of the braiding machine to the proper positions. This is a slow process and making a Hoytether kilometers long with these machines would be very time consuming and expensive. The composites division of Albany International (Albany International Research Company, 777 West Street, Mansfield, Mass.) also produces a 3-D braiding machine. This machine uses modular braiding components that are assembled breadboard fashion on a large wall.
While braiding is often used in producing high-strength cables and composite materials, most textile materials are fabricated using knitting and weaving processes. Two such techniques that have the potential for fabricating multiline tether structures include Raschel knitting and 3-D weaving. 3-D weaving requires the use of a cross fiber that adds to the weight, but not to the strength of the tether. The Fukui Company of Japan manufactures knitted netting by a proprietary technique (Fukui Net Company, Ltd., PO Box 119, Island View Drive, Golden Lake, Ont., Canada K0J 1X0.) Fukui produced a sample Hoytether wherein the primary lines were generated as a 4-strand knitted structure and the secondary lines were created by exchanging yarns from one primary to another. Although the secondary lines were interwoven into the primaries, they were exchanged between primaries in such a way that some of the interconnections were not slip-proof joints. As a result, a primary line cut tended to cause this knitted structure to collapse. It may be possible to use Raschel knitting to produce joints that are slipless and could be used to practice the present invention.
Tether Systems
The prior art teaches the use of tethers in space applications. U.S. Pat. No. 5,163,641, issued on Apr. 9, 1990 to Yasaka, teaches the use of a powered spacecraft connected by a tether to a satellite. This tether is disconnected to change the state vector of the satellite. The state of the art of energy and momentum transfer using space tethers is discussed in Ivan Beckey""s article xe2x80x9cTethering a new Technique for Payload Deploymentxe2x80x9d, Aerospace America, March 1997, at pages 36-40. Beckey concludes, xe2x80x9cTethers can perform the same functions as propulsive upper stages of direct payload injection, but at lower weight and cost per pound.xe2x80x9d U.S. Pat. No. 4,923,151, issued Mar. 1, 1988 to Roberts, Wikinson and Webster, teaches a tether power generator for earth orbiting satellites. U.S. Pat. No. 4,580,747, issued Mar. 15, 1983 to Pearson, teaches use of a long tether extending downward into the atmosphere from a satellite. The state vector of the satellite is changed by forces acting on a lifting body connected to the end of the tether. U.S. Pat. No. 4,824,051, issued Jan. 12, 1987 to Engelking, teaches passing an electric current through a conductive tether attached to a satellite to provide propulsive force to alter the orbit of the satellite. U.S. Pat. No. 5,082,211, issued Jan. 21, 1992 to Werka, teaches use of a tether to deorbit space debris. U.S. Pat. No. 4,727,373, issued Mar. 31, 1986 to Hoover, teaches an orbiting stereo imaging radar system having two spacecraft in synchronous parallel orbits connected by a tether.
The following references are illustrative of the current state of the art in space tethers:
1. Paul A. Penzo and Paul W. Ammann. Tethers in Space Handbookxe2x80x94Second Edition. NASA Office of Space Flight, NASA Headquarters, Washington, D.C. 20546. See the hundreds of references in the 33 page bibliography at the end of the handbook.
2. Joseph A. Carroll, xe2x80x9cSEDS Deployer Design and Flight Performance.xe2x80x9d paper WSEDS-A-1 at the 4th International Conference on Tethers In Space, Washington D.C., April 1995.
3. Robert L. Forward, Failsafe Multistrand Tethers for Space Propulsion, Forward Unlimited, P.O. Box 2783, Malibu, Calif. 90265, July 1992, Final Report on NASA Contract NAS8-39318 SBIR 91-1 Phase I.
4. Robert L. Forward and Robert P. Hoyt, Failsafe Multistrand Tether SEDS Technology Demonstration, Final Report on NAS8-40545 with NASA/MSFC (Jun. 14, 1995).
5. Robert L. Forward and Robert P. Hoyt, xe2x80x9cHigh Strength-to-Weight Tapered Hoytether for LEO to GEO Payload Transferxe2x80x9d Final Report on contract number NAS8-40690 with NASA/MSFC Jul. 10, 1996).
6. Hans Moravec. xe2x80x9cA Non-Synchronous Orbital Skyhookxe2x80x9d. J. Astronautical Sci., 25(4):307-322, October-December 1977.
7. Robert L. Forward. xe2x80x9cTether Transport from LEO to the Lunar Surfacexe2x80x9d. In AIAA/SAE/ASME/ASEE 27th joint Propulsion Conference, Sacramento, Calif., June 1991. AIAA Paper 91-2322.
8. Joseph Carroll. xe2x80x9cPreliminary Design of a 1 km/sec Tether Transport Facilityxe2x80x9d, Technical Report, Tether Applications, 2603 Crosshaven Lane, San Diego, Calif. 92139, March 1991. Final Report on NASA Contract NASW-4461.
The structure, fabrication and use of a tether having a plurality of primary (load bearing) lines and knotless, slipless interconnected, normally slack secondary (non-load bearing) lines, called a xe2x80x9cHoytetherxe2x80x9d, is the single general inventive concept of the present invention. This Hoytether is the special technical feature common to all the claims. It is a contribution to the art that each embodiment of the invention disclosed herein, considered as a whole, makes over the prior art.
The technical problem solved by this invention is catastrophic failure of single line tethers due to impact of debris, materials defects or electrical arcing (for electrodynamic tethers). The SEDS and TSS experiments cited above highlighted this technical problem. This type of failure is unavoidable due to the xe2x80x9c1/exe2x80x9d failure curve which physics demands of any single line tether. Tether failures on these missions cost hundreds of millions of dollars and wasted years of research. The conventional solution to this problem is shown by the TiPS mission, also cited above. For this mission the single line tether was xe2x80x9cfluffed upxe2x80x9d to xe2x80x9cgive it a larger cross section to improve its resistance to debris and small micrometeoroidsxe2x80x9d. The invention disclosed and claimed herein provides a radically different, technically superior and nonobvious solution to this problem.
How superior is a floytether to a single line tether? Theoretically, the increase in Hoytether lifetime over the lifetime of a single-line tether for the same mass is approximately proportional to the number of interconnection levels in the Hoytether and inversely proportional to the number of Hoytether lines to the xc2xeths power. The number of interconnection levels typically range from 1000 to 1,000,000, while the number of lines varies from 2 to 12. Because of the large number of interconnection levels, Hoytethers can be expected to survive 10 to 1000 times longer than a single-line tether of the same mass in the same environment. FIG. 1 shows the small impactor survival probability of equal weight single line and Hoytethers for a low-load mission.
Another benefit of the invention is that the survival probability curve of the Hoytether as a function of time does not conform to the standard xe2x80x9c1/e decayxe2x80x9d shape of a single-line tether. The Hoytether maintains a high level of survival probability, about 99 percent, until it nears its xe2x80x98lifetimexe2x80x99. Its survival probability then drops rapidly to zero. A detailed mathematical analysis of the difference between Hoytether and single tether survival probabilities is given in Appendix E xe2x80x9cSmall Impactor Survival Probabilities of Hoytethersxe2x80x9d and Appendix F xe2x80x9cLarge Orbital Debris Survival Probabilities of Hoytethersxe2x80x9d, both in NASA Reportxe2x80x941995, which is incorporated into this disclosure by reference. The resulting cut probability with time for the Hoytether has a pure xe2x80x9cbingo curvexe2x80x9d shapes. In a bingo game, at least five numbers must be called before anyone can win, and usually many numbers have to be called before one of the bingo cards gets five in a row. In the present invention, at least three cuts must happen at the same level before any failure occurs, and many cuts have to be made before any one of the levels has all three lines cut. The bingo curve has the property that the probability of survival stays very high for periods short compared to the lifetime. The probability of survival is greater than 99.9% for periods shorter than 10% of the lifetime. This is much better performance than the 1/e curve of a single line tether, where the probability of survival is only 90% at 10% of the 1/e lifetime.
For a NASA statement confirming this conclusion see the text and charts at:
The present invention is discussed in this disclosure in terms of its space applications. It should be understood, however, that the Hoytether is useful in any application where the tether must be operated without failure for long periods of time in hostile environments and/or safely when the load is near the ultimate strength of the tether material. One example would be the mooring or towing lines used with large ocean going ships.